It might depend on what you want to do but if you can set a Siri shortcut for an action in Overcast (Settings > Siri Shortcuts), you can call it by voice. There are a lot of available options:
As shown above, I have one set to resume playing whatever I was listening to in Overcast that I use in the vehicle a lot by shouting out “play Overcast.” Well, I used to use that a lot, but in iOS 13 I created an Automation in Shortcuts that fires when I plug into CarPlay since 99% of the time I want to listen to podcasts and with iOS’s whacky resume behavior you never know what will start playing when you plug-in:
Voice Boost and Smart Speed (also available in others, like Castro) are the reason I don’t simplify to the Podcasts app.